Signaling tones including one or more predetermined frequencies are used extensively throughout telecommunications systems. A familiar example is a call progress signaling tone, which is a supervisory signal generated in a telephone central office during the time interval between initiation of a telephone call and termination of the call. Typical call progress tones include audible ring, busy, and reorder. Although call progress tones are usually intended for aural recognition by the calling party, a business machine can be advantageously arranged to take appropriate actions if the machine is able to selectively detect and identify a tone. Accordingly, with the increasing use of digital transmission terminals, a need naturally arises for economical and reliable arrangements for tone detection in a digital environment.
Unfortunately, the signaling tone is often corrupted by various sources of noise. For example, low frequency inductive switching transients may distort boundaries of a pulse tone. Further, Gaussian, impulse, or 60-hertz noise may even mask the presence of low-level tones. Conversely, noise may mask the absence of a signaling tone. Furthermore, speech may imitate an inband signaling tone and thereby produce false signaling information. These unfortuitous aspects of tone detection are even more aggravated by the current absence of a more quantitative description of the various noise sources.
Existing analog tone detectors avoid some of these problems by using bandpass or band elimination filters to first separate signals from noise sources in the frequency domain. The analog filters are usually coupled with an energy detector and comparator, the outputs of which are advantageously arranged to determine whether a tone signal is present. The extensive use of the frequency domain for analog detection is based in part on the advanced state, and consequently the low cost, of analog filter technology. By way of contrast, however, digital filters are typically substantially more expensive. As a result, the frequency domain approach to digital tone detection in a large telecommunications system is economically restrained.